CSC 453

Operating Systems

December 31, 2025

Who am I?

https://pschmitt.net
Academics
- PhD at UCSB
- Researcher (summers) at ICSI
- Postdoc then research faculty at Princeton
- Some other stops… USC, Stanford, UH
Privacy startup co-founder & CEO, measurement startup COO
Main focuses
- Integrating privacy and security into systems that we all use
- ML for network management
- Connectivity / privacy in challenging environments

This class

Too much to cover in one term: survey of important topics
Course Canvas: https://canvas.calpoly.edu/courses/168889
Syllabus:
Waitlists…

Who are you?

Name / Major / Year
Where do you identify as your hometown?

How to succeed in this class

Turn things in on time
Attend class
- … on time. Lectures will start at 10 after the hour
Ask questions
- I’m very flexible about how much we cover this semester
- I would rather teach less and have everyone understand it
- Our back-and-forth during class is the one of the few indicators I have of how much you are absorbing
Talk to me if you are struggling

What are we doing here?

How many of you have participated in OS development?
How many of you regularly program in languages that use operating system abstractions directly?
And C is a decreasingly popular language!
So why study operating systems? Why is this class even offered? Why is it required?

Why take this class

You are required to do so in order to graduate
Reality: this is how computers really work, and as a computer scientist or engineer you should know how computers really work
Ubiquity: operating systems are everywhere and you are likely to eventually encounter them or their limitations
Beauty: operating systems are examples of mature solutions to difficult design and engineering problems. Studying them will improve your ability to design and implement abstractions

Course progression

Introduction to operating system abstractions and structures
Abstracting and multiplexing:
- CPU: interrupts, context, threads, processes, processor scheduling, thread synchronization, deadlocks
- Memory: memory layout, address translation, paging and segmentation, address spaces, translation caching, page fault handling, page eviction, swapping
- File systems and storage: disk scheduling, on-disk layout, files, buffer cache, crash and recovery
- Security (time permitting)

Background

Intro

What is an operating system?
What do they do?
How did OSes evolve?
How do OSes manage access to resources?

What is an operating system?

Operating System:
- A computer program that
- multiplexes hardware resources and
- implements useful abstractions.
The OS is just another computer program. Has the highest privilege. The core of the OS consists of the kernel
Multiplexing allows multiple people or programs to use the same set of hardware resources - processors, memory, disks, network connection - safely and efficiently.
Abstractions simplify the usage of hardware resources by organizing information or implementing new capabilities.

OS abstractions

Abstractions simplify application design by:
- hiding undesirable properties,
- adding new capabilities, and
- organizing information
Abstractions provide an interface to application programmers that separates policy—what the interface commits to accomplishing—from mechanism—how the interface is implemented.

What are the abstractions?

CPUs
- Processes, threads
Memory
- Address space
Disk
- Files

Example OS abstraction: file systems

What undesirable properties do file systems hide?
- Disks are slow!
- Chunks of storage are actually distributed all over the disk
- Disk storage may fail!
What new capabilities do files add?
- Growth and shrinking
- Organization into directories, searchability
What information do files help organize?
- Ownership and permissions
- Access time, modification time, type, etc.

Abstraction tradeoffs - discussion

Identify undesirable properties hidden by, new capabilities added, and info organization provided with these abstractions:
- Process / threads
- Address space

Processes/Threads

Hiding undesirable properties:
- Example: The process abstraction hides the complexity of CPU scheduling and context switching. Applications don’t need to manage the low-level details of how the CPU switches between different tasks. Similarly, the thread abstraction hides the intricacies of managing multiple execution paths within a single process, allowing developers to focus on the logic of concurrent tasks.
Adding new capabilities:
- Example: Processes provide isolation between different applications, ensuring that one misbehaving application doesn’t affect others. Threads within a process allow for parallel execution of tasks, improving performance and responsiveness. Additionally, operating systems often provide inter-process communication (IPC) mechanisms, enabling processes to coordinate and share data.
Organizing information:
- Example: The process abstraction organizes the execution environment by encapsulating the code, data, and resources needed for a program to run. Threads further organize execution by dividing tasks within a process into smaller, manageable units. This organization helps in structuring complex applications and improving their maintainability.

Memory Address Space

Hiding undesirable properties:
- Example: The memory address space abstraction hides the physical memory layout from applications. Programs use virtual addresses, which the operating system maps to physical memory locations. This abstraction also hides the details of memory fragmentation and allocation, simplifying memory management for developers.
Adding new capabilities:
- Example: Virtual memory allows applications to use more memory than physically available by swapping data to and from disk. This capability enables larger and more complex applications to run on systems with limited physical memory. Additionally, memory protection mechanisms prevent one process from accessing the memory of another, enhancing system stability and security.
Organizing information:
- Example: The memory address space abstraction organizes memory into segments such as code, data, heap, and stack. This organization helps in managing different types of data efficiently and ensures that memory is used in a structured manner. For instance, the stack is used for function calls and local variables, while the heap is used for dynamic memory allocation.

What isn’t clear? Comments? Thoughts?

Abstraction pros / cons

Advantages of OS providing abstractions?
- Allow applications to reuse common facilities
- Make different devices look the same
- Provide higher-level or more useful functionality
Challenges?
- What are the correct abstractions?
- How much should be exposed?

OS design requirements - what do we need?

Convenience, abstraction of hardware resources for user programs
Efficiency of usage of CPU, memory, etc.
Isolation between multiple processes
Reliability, the OS must not fail
Other:
- Security
- Mobility

So, what is an operating system?

Another view: resource manager - shares resources “well”
Advantages of the OS managing resources?
- Protect applications from one another
- Provide efficient access to resources (cost, time, energy)
- Provide fair access to resources
Challenges?
- What are the correct mechanisms?
- What are the correct policies?

Resources are managed via services

Program Execution (loading, running, monitoring, terminating)
Performance (optimizing resources under constraints)
Correctness (overseeing critical operations, preventing interference)
Fairness (access to and allocation of resources)
Error detection & recovery (network partition & media failure)

Services (cont’d)

Communication (inter-process, software-to-hardware, hardware-to-hardware, system-to-system, wide-area)
I/O: reading & writing, support for various mediums, devices, performance, and protections
Data Organization (naming), Services (search) & Protection (access control)
Security (isolation, enforcement, services, authentication, accounting and logging, trust)
User interfaces (command-line, GUIs, multiple users)

Each service has challenges and tensions

Example 1: We have limited RAM, and we want to run more programs that can be stored.

How do we allocate space?
Who stays?
Who goes?
What if we’re wrong?
What if the system is under extremely heavy load?
Is there a way to predict the future?

Each service has challenges and tensions

Example 2: We have two process (producer / consumer); how do they communicate?

Message passing? Shared memory?
How do they synchronize?
- How to we prevent over-production? Over-consumption?
- Context-switching?

What isn’t clear? Comments? Thoughts?

SLOsh

Signal handling

General
Signal handler for SIGINT
Parent vs child
sigaction

General

Signals are async. They can happen at any time
Certain functions are async-safe, others are not
- printf() contains internal buffers and state; an interruption can lead to corrupted buffers

Handling SIGINT

write() a newline or whatever to signify the signal was caught.
If there is not a child running, write() a new prompt
Why not just use display_prompt()?
- If it’s using write() it could be fine. If it’s using printf() it isn’t

Parent vs child

The child should immediately reset signal handling to the default
Look up signal()’s man
```
man 2 signal
```

Sigaction

sigaction() is newer than signal()
Offers more control and is more consistent across systems
Allows specifying flags
- SA_RESTART (man 7 signal)
  - Without it, syscalls return with EINTR error if they are interrupted by a signal
  - With it, most syscalls automatically restart
Is the POSIX-compliant approach
EINTR can still happen, it’s wise to check for it and handle it
- When reading input from user